Prairie Dog,
It doesn't have what I would call a "tuned" sound. It doesn't have that nice rumble like some of the aftermarket exhaust systems, and even less so than the OEM. But, at 75-80 mph the wind noise inside the cab is louder than the exhaust. Every so often I have an 11 hour trip to make at 75-80, and a constant drone would be unacceptable. Even at slower speeds, I'm not really aware of the exhaust noise.
Compared to the stock exhaust, it definitely sounds like it's moving more air volume at a slightly higher pitch, even at idle.
Honestly, I'd prefer a little more deep rumble, but the straight-flow system does what I need it to, with respect to lower EGT's, and without the highway drone.
Note in my truck signature that I do have sound dampening in the roof, firewall, doors, floorboards, and rear wall. I don't think that had much effect, if any, on the perception of exhaust noise inside the cab. The biggest difference I noted from the sound dampening was a reduction in tire noise and vibration (I run 315's) after doing the rear cab wall. I tested the sound dampening before and after with a decibel meter, and only saw a decrease of a few db's at idle through 80mph. With out the sound dampening, I don't think you'll hear much difference over stock inside the cab with the Donaldson.
Scott